1. Field of the Invention
This invention relates to the transistor(s) used in electrical power switching circuits and, more particularly to a drive circuit which allows for a controlled turn off of the power transistor without effecting transistor turn on.
2. Description of the Prior Art
In many applications such as power conversion circuits, it is necessary to operate transistors as power switches to interrupt substantial current flows. In the presence of such large current flows, a transistor tends to accumulate a charge in the vicinity of the base region which charge interferes with the rapid turn off of the transistor switch. Turn off of the transistor will be delayed until such time as the charge stored in the transistor's base region is substantially removed. This period of time is known as the storage time.
There have been a number of techniques proposed in the prior art for removing the stored charge during turn off of the transistor. Common to many of these techniques is the attempt to try to remove the stored charge from the base region as quickly as possible. It has now been found that if the charge is pulled out of the base region too quickly the collector current will be caused to flow in a narrow path or channel. In effect an attempt to rapidly remove the charge from the transistor's base region in order to expedite transistor turn off results in the transistor's collector current flowing through a narrower collector-emitter junction than the collector current would ordinarily flow through. Thus the attempts to expedite charge removal from the base region place a strain on the transistor which may result in failure of the device.
The attempt to expedite charge removal from the base region also results in unneeded power loss in the transistor during turn off. Fast removal of the charge from the base region causes the collector current to fall towards zero with a tail or fillet which may extend for an appreciable period of time. Simultaneously therewith as the transistor is driven out of saturation towards cutoff the collector to emitter voltage rises substantially. The combination of the increasing collector to emitter voltage and the slowly decaying tail on the collector current results in power losses. These losses in turn may cause excessive heating of the transistor thereby resulting in device failure.
It was therefore recognized that rapid removal of the base charge may not be the way to expedite turn off of the transistor. In an article entitled "Base Circuit Design for High-Voltage Switching Transistors Power Converters" by W. Hetterscheid and published in the September/October 1980 edition of Power Conversion International pages 39 to 46 the turn off as well as the turn on behavior of high voltage power transistors used in switched mode power supplies is discussed. In this article various techniques for turning off such transistors are discussed. The article indicates that an improvement in turn off behavior can be obtained by slowing down rather than speeding up removal of charge from the transistor's base region. In particular in FIG. 4C thereof a turn off circuit using an inductor in series with the transistor base is shown. It is further stated that the fall times and the time for base charge removal are increased when the temperature of the junction is increased.
The article then describes an optimum base drive circuit which is shown in FIG. 10A thereof. This circuit while continuing to use the inductor in series with the switching transistor's base also includes resistive and capacitive elements which are said to be needed in order to obtain optimum turn on of the transistor. It is also suggested that a resistor be connected between the transistor's base and emitter to eliminate parasitic oscillations (ringing). Thus while the prior art has come to recognize that an inductor may give superior results in helping to remove stored charge from the base of the transistor it has been believed necessary to compensate for the effects of the inductor during transistor turn on.
It was then recognized that it would be far more desirable to remove stored charge from the base region of a transistor in a manner so as to avoid the power losses associated with the prior art techniques and also to avoid the introduction of unnecessary circuitry during turn on of the transistor. It was further recognized that it would be desirable to provide this removal of stored charge from the transistor's base region in a manner such that the variation in transistor operating characteristics which often occurs in transistors of the identical type would have no effect on the operation of the circuit in which the transistor is used. It was further recognized that removal of stored charge from the transistor's base region during turn off should be provided in a manner such that power losses are minimized and in a manner which is substantially insensitive to the operating temperature of the transistor.
In accordance with the present invention a controlled turn off of a transistor operating in its saturated conducting mode is obtained by the use of a source of constant voltage which is directly coupled to the base of the transistor. The drive transformer associated with the transistor uses a two core arrangement, consisting of linear and non-linear cores wherein turn off is initiated by shorting the non-linear core. The linear core then supports the voltage to give a slowly decreasing base current which linearly changes with time. Turn off of the transistor is accomplished without the harmful effects associated with prior art turn off techniques. The circuit of the present invention also allows turn on of the transistor to be accomplished without the necessity of introducing additional circuit elements.